How are we to account for social norms and their normativity? Why do we believe that in certain situations that we ought to act honestly or politely or that we ought to cooperate with others? The phenomenon of social norms represents a serious challenge for individualistic social theories. Trying to cope with the weaknesses of these accounts, alternative proposals have been offered. This paper evaluates Margaret Gilbert's theory of social norms. Focusing on the social rationality of individuals, Gilbert tries to explain the normativity of social norms in the ways individuals are part of a social setting. Her account has been considered as a promising alternative to rational-choice approaches (see, for example, Elisabeth Anderson 2000). Nevertheless, while Gilbert's general account of social phenomena has been widely discussed, her account of social norms has not been analysed thoroughly. In the following, I argue that Gilbert does not adequately capture the phenomena of social norms and their normativity.